Replaced set with set1 instructions.

 * Replace `_mm_set_epi32` with `_mm_set1_epi8` fuction in
   `calc_checkdigit` and `utf8_strlen_sse2` functions for
   8-bit digit bounds ('0' and '9').
 * Replace `_mm_set_epi32` with `_mm_set1_epi16` function in
   `calc_checkdigit` for 16-bit hi and lo masks.
Replaced sub with xor.

 * Replace `sub_epi8` with `xor_si128` in `calc_checkdigit` function.
 * Rectify comments.